Modern formulated industrial oils typically use corrosion inhibitors to prevent the formation of rust on steel parts. It is well known that certain types of carboxylic acids act as corrosion inhibitors. In particular, alkylated succinic acids and their mono ester derivatives are well known in the art as corrosion inhibitors for hydraulic fluids and gear oils.
For example, U.S. Pat. No. 4,409,411 describes the reaction product of an alkyl substituted succinic anhydride and a polyol as a corrosion inhibitor for fuels and lubricants. U.S. Pat. No. 4,165,292 describes a mixture of a basic amine compound with a succinic acid derivative as a corrosion inhibitor for lubricants.
In addition, several acylated aspartic acid derivatives, such as those described in U.S. Pat. Nos. 5,275,749 and 4,462,918 and European Patent No. 86 513 are known in the art.
It is also known in the art to employ compounds possessing both carboxylic acid and hydroxyl functional groups as corrosion inhibitors for a variety of applications. For example, U.S. Pat. No. 5,055,230 discloses the use of a composition containing hydroxy, ether and carboxylic acid functionality as a corrosion inhibitor for aqueous and oil based systems. U.S. Pat. No. 4,892,671 describes aminoethyl-2-propanol derivatives as corrosion inhibitors for ferrous metal. U.S. Pat. No. 5,254,277 discloses a combination of bis(hydroxyethyl)tallow amine and an alkyltallowdiamine dicarboxylate as a corrosion inhibitor for polyol ester fluids. U.S. Pat. No. 4,957,641 describes alkyloxyhydroxy fatty acids and salts thereof as useful corrosion inhibitors in oils and oil-containing emulsions. U.S. Pat. No. 4,233,176 discloses a solution consisting of monobasic acids, a lubricant, a mixture of amines, and water as a corrosion inhibitor system for metals prior to painting. U.S. Pat. No. 4,743,388 teaches a complex amide carboxylate anti-corrosion additive for metal working fluids.
Formulated hydraulic fluids, gear oils, and circulating oils typically use mineral oil base stocks as the major lubricating component. These base oils are used in conjunction with various additives such as antioxidants, antiwear agents, and corrosion inhibitors to obtain a final formulation which meets the performance requirements of the particular industrial oil application.
In recent years there have been several major advances in the refining of mineral base oils. Process steps such as severe hydrotreating remove any unsaturation and impurities from the oils. The result is a base oil with improved thermal stability. In addition, there is now increased use of non-polar synthetic base oils such as poly-alphaolefins as base oils for formulated lubricants.
As the performance of hydraulic oils, gear oils and other industrial oils become more severe, industrial lubricant formulators have begun to use severely hydrotreated base oils and synthetic base oils such as poly-alpha-olefins because of their low cost and their improved thermal stability relative to conventional mineral oil base stocks. However, there are certain disadvantages to the use of these highly refined base oils. Most of the commonly used lubricant additives such as amine antioxidants, phenolic antioxidants, antiwear additives and corrosion inhibitors have decreased solubility in hydrotreated base stocks and/or poly-alpha-olefins. More importantly, such changes in additive solubility in many cases negatively effects the performance of the additives (heretoafter referred to as additive response). In general, the additive response decreases significantly in the highly refined base oils or non-polar synthetic base oils. This phenomenon may be more pronounced for those additives which function as surface active agents.
It has unexpectedly been discovered that the ashless compositions of matter of the present invention have excellent solubility in severely hydrotreated base oils and non-polar synthetic base oils such as poly-alpha-olefins as well as traditional mineral oils and, when incorporated in relatively small amounts, dramatically improve the rust protection properties of finished hydraulic fluids formulated with highly paraffinic and severely hydrotreated mineral base oils, or non-polar synthetic base oils such as poly-alpha-olefins.